Controlling leaf diseases in seed corn in 2002

The time is fast approaching when seed producers need to be looking for early symptoms of leaf diseases in seed corn. Eyespot (Aureobasidium zeae), common rust (Puccinia sorghi), gray leaf spot (Cercospora zeae-maydis), and northern leaf spot (Bipolaris zeicola, also known as Helminthosporium carbonum) are diseases that can cause losses in seed corn production and sometimes need to be controlled with a fungicide application. In addition, Northern leaf blight (Exserohilum turcicum) seems to be making a "comeback" in some areas.

The benefits of foliar fungicides on seed corn have been researched for a number of years at Iowa State University. Protecting susceptible inbreds with a fungicide has proven to be very profitable. Fungicide options have been changing; last year, Quadris (Syngenta Crop Protection) was first labeled for corn. This year, another new fungicide, Stratego (Bayer Corp.), is labeled (see June 17, 2002, ICM article). Quadris and Stratego, which both have active ingredients in the strobilurin family, offer some advantages over former standard, Tilt. Generally, Quadris, and Stratego provide superior disease control compared with Tilt and yield increases that are similar or better than those we have seen with Tilt. Three other fungicides may be used on seed corn; they differ in their type of activity (contact versus systemic), spectrum of disease control, and application requirements. The table provides some comparisons of the fungicides most commonly used on seed corn.

For now, guidelines for foliar disease control are based on scouting, relative susceptibility of the seed parent inbred, and weather considerations. We are currently researching methods to predict gray leaf spot so that fungicide application decision-making can be improved. In general, the most profitable results have occurred when sprays are initiated before detasseling. However, most of these results were obtained before Quadris was available. Given the efficacy of Quadris and the label allowing post-silking applications, there are situations in which a single post-silking application of Quadris is the best choice, depending on the scouting results.

The following guidelines should be followed for the control of leaf diseases in seed corn:

Do not plant seed corn in a field where corn was the previous crop, unless absolutely necessary.

Know the susceptibility of the inbreds you are growing because the more resistant inbreds rarely need a fungicide. The more susceptible the inbred, the more likely fungicide use will be profitable.

Scout fields early, when plants are approximately V6-V8. Observe at least 100 plants throughout the field. Record the average number of pustules or lesions per plant, disregarding the bottom three leaves.

Scout every 1-2 weeks, depending on weather and susceptibility. Interval should be shorter in wet, cool weather and on the most susceptible inbreds, longer in hot, dry weather and on more resistant inbreds.

When there is an average of 1-2 pustules or lesions per plant, and weather is favorable for disease (moderate temperatures and frequent rains or dews), begin spraying susceptible inbreds. Remember that fungicides are most effective when sprayed before infection takes place, so you must consider the weather forecast as well as previous weather.

Leave an unsprayed area for comparison. There is always a temptation to protect everything, but an unsprayed check provides valuable information on the effects of spraying.

Follow label instructions for rates and spray intervals. Because symptoms of infection for some diseases do not appear for 10-20 days, some infections that occurred before you sprayed may continue to appear after you spray (less likely with systemic fungicides).

Your decision to spray again should be based on the label instructions, weather, and disease development in unsprayed areas. With the systemic products available today, more than two applications would be unlikely.

If diseases have not appeared before tasseling, spraying is probably unnecessary.

Always check the label of any pesticide to confirm that it is registered for the intended use and to be sure that all label requirements are being fulfilled.

Commonly used fungicides for controlling corn leaf diseases.

Quadris

Stratego

Tilt

Penncozeb

Bravo

Active ingredient

Azoxystrobin

Trifloxystrobin

and Propiconazole

Propiconazole

Mancozeb

Chlorothalonil

Activity

Systemic

Systemic

Systemic

Contact

Contact

Spectrum

Rust, gray leaf

spot, northern

leaf blight,

northern leaf

spot

Rust, gray leaf

spot, eyespot,

northern

leaf blight,

northern leaf

spot

Rust, gray leaf

spot, eyespot,

northern

leaf blight,

northern leaf

spot

Rust, gray leaf

spot, northern

leaf blight

northern leaf

spot

Rust, northern

leaf blight,

northern leaf

spot

Spray interval

7-14 days

7-14 days

7-14 days

4-7 days

4-7 days

Preharvest interval

7 days

Silking

30 days*

40 days

14 days

Feeding

Yes

Yes

Yes*

Yes

No

*If Tilt is applied after silking, crop residue cannot be fed to livestock.

This article originally appeared on pages 117-118 of the IC-488(14) -- June 24, 2002 issue.